Many battery powered products require the use of multiple cell battery packs with the cells connected in series to achieve a high enough voltage for proper operation. The rechargeable type of battery packs need to be recharged periodically so that the powered operation can continue. And, special care needs to be taken to avoid damage to the battery cells during charging and operation. Two typical damages to battery cells are overcharging and deep-discharging.
Overcharging may cause severe damages to battery cells, and may even become safety concerns. Overcharging lithium-ion or lithium polymer batteries, for example, may cause thermal runaway, and the high temperature developed may lead to cell rupture. Fire hazards have been reported during charging as extreme cases. Therefore, much attention has been paid to battery overcharging and solutions have been designed to avoid battery damage and safety issues. The typical charging system developed applies one charger to charge all cells connected in series in a battery pack. Since all cells are not manufactured the same, some cells may charge faster than others. As such, charging a battery pack with a plurality of cells with a single charger can lead to overcharging some of the cells.
U.S. Pat. No. 4,079,303, issued to Cox on Mar. 14, 1978, discloses a two step charging system to charge all battery cells connected in series at an initial charging rate to a predetermined voltage threshold, followed by an equalization procedure for charging each of the cells with controlled and equal voltage to fully charged state. The disadvantage of such a system is that the equalization phase is based on a conservative estimate of a predetermined charging voltage threshold. And the charging process can be time consuming when the battery pack contains many cells.
A common practice of using a single charger to charge a battery pack of a plurality of cells is to use shunt circuit to bypass the charging circuit of each individual cell when it is fully charged to avoid overcharging and over heat. U.S. Pat. No. 6,388,424B1, issued to Hidaka et al on May 14, 2002, teaches a system for charging a plurality of lithium-ion battery cells connected in series. And, each of the cells has a shunt circuit connected in parallel with the cell. A comparator compares the charging voltage of each cell with a reference voltage. When the charging voltage is higher than the reference voltage, a switch is activated to direct the electricity to the shunt circuit for the cell. Although the invention is trying to supply the surplus energy to the next cell in line, this system inevitably causes energy waste for charging which is not friendly to the environment.
Another damage that can happen to battery cells is deep-discharging. And this is especially true for lithium-ion and lithium polymer batteries. When a charged battery pack of a plurality of cells is connected to a load, each cell is gradually discharged, and the cell voltage declines. When a cell is discharged below a defined low voltage threshold, further discharging may damage it. After that, the cell may have degraded storage capacity. For example, a certain lithium-ion battery cell of the LiFePO4 variety should not be discharged below 2.5 volts to avoid deep-discharging damage. It is important, therefore, that care is taken to remove the battery load before the low cell voltage limit is reached.
Therefore, it is important to detect the voltages of the cells in a battery pack during charging or operation to effectively avoid over charging or deep-discharging. In a pack of a plurality of battery cells connected in series, cell voltage can be estimated by measuring the overall pack voltage and dividing it by the number of cells in the pack. This is only accurate, however, if the cells are nearly identical, which is rarely the case due to variations of components and manufacturing processes. It is apparently advantageous to measure the voltage of each of the plurality cells due to the type of uncertainties. This involves making differential voltage measurements in the presence of common mode voltages that are dependent on each individual cell's position in the pack. The measurement results are normally analog or digital signals that are referenced to a common voltage, typically the pack negative terminal. For a battery pack, there exist a predetermined high voltage limit and a predetermined low limit for each of the plurality of cells. During charging, a cell having its voltage reaching the predetermined high voltage limit will cause the charging of the cell to stop. During operation, any cell in the pack having voltage reduced to the predetermined low voltage limit will trigger load removal. For a battery pack of the LiFePO4 type cells, for example, a measurement of voltage below 2.5 volts for any cell in the pack will trigger the load removal. Since actual measurement of each cell is far superior to estimated voltage, cell damage due to overcharging and deep-discharging can be effectively avoided. Hence cell life is maximized and safety issues are prevented.
Many conventional ways exist to measure the voltage of each cell in a battery pack. Most of these approaches are expensive, mainly due to the large common mode voltages involved when there are many cells in the pack, and complex due to the wiring necessary to each cell. Therefore, there is a need for a simple and inexpensive way to detect voltage for each cell in a battery pack containing a plurality of cells.